Dogs are popular and ubiquitous pets, making them the commonest large carnivore in the world. We argue that the negative environmental effects of pet dogs are multifarious, ranging from direct predation and disturbance of wildlife to indirect modifications of wildlife behaviour due to their scent marks, faecal and urine pollution, deposition of veterinary insecticides into the environment, and carbon footprint of the pet food industry. The mitigation of these effects lies largely in the responsible behaviour of dog owners.